The Sídhe (pronounced "Shee") are an immortal race of Avalon, the Island of Youth, and they appear as small blue humanoid winged creatures. Gaius describes them as "masters of enchantments and a cruel race of beings." The Sidhe had kings who were called Sidhe elders, they were served by Pixies and could live for 1000 years.

The Sidhe banished Aulfric and Sophia from Avalon giving them human bodies and a mortal existence, as punishment for Aulfric killing another Sidhe. Aulfric wished to allow Sophia access to return to Avalon and convinced her to enchantArthur Pendragon so that they might sacrifice him to the Sidhe, who required the soul of a mortal prince as payment for immortality for Sophia. Merlin turned up at the last minute and saved Arthur by attacking and killing both Aulfric and Sophia with Sophia's Sidhe staff (The Gates of Avalon).

When Elena was 20, Godwyn and his old friend Uther Pendragon organised an arranged marriage between Arthur and Elena. Merlin soon discovered the truth but when Grunhilda informed the Sidhe elder of this he went to kill Merlin in his sleep. Merlin, however, was able to destroy him using the Sidhe staff that previously belonged to Sophia. Gaius then created a tonic to get the Sidhe out of Elena and Merlin killed Grunhilda, along with the faerie (The Changeling).

They were mentioned by Gaius when Merlin asked if their was a way to save Arthur and Gaius replied that it would take magic as old as the dragons to heal and told Merlin The Sidhe could save Arthur (The Diamond of the Day: Part 2).

Sidhe (pronounced 'shee') literally means "people of the (fairy) hills". It is the Gaelic name for the fairies in both Ireland and the Highlands of Scotland. Usually these fairies are attracted to those who are beautiful as well as wealthy.

The Sidhe are very powerful creatures, capable of casting very strong spells, and are described as "masters of enchantments". Many of their kind use staffs to help focus their powers when casting spells. This can range from enchantments to bolts of energy; other wielders of magic can also use these staffs for the same purposes.

They practice their own kind of magic, known as Sidhe magic. They also have the ability to implant one of their own kin inside a human, during the human's infancy. The Sidhe would continue to live inside of the person, till about twenty years later, when it would emerge and possess the person, taking complete control over them.

They can also fly, and are able, with enough speed, to keep their true form hidden to normal eyes, thus appearing only as a glowing sphere. They can also curse members of their own kind to have a mortal existence, and a mortal body, but they do not appear to be able to remove their powers.

Although not truly immortal, the Sidhe are very close because they can live for thousands of years; because of this, they are as ancient as the Dragons. They can apparently only be killed by strong magic. If cursed to a mortal existence, they find the experience of almost dying unnerving.

It was hinted by Gaius that the Sidhe may have powerful healing Magic, as he told Merlin that is would take Magic as old as the Dragons to heal Arthur and believed that The Sidhe could heal Arthur.

In Gaelic (Irish and Highland Scottish) mythology and folklore the Sídhe are also known as Aos Sí, and sometimes as the Túatha De Danann. They are similar to the Elves of Norse mythology (and as, to some extent, those portrayed in Tolkien's works on Middle Earth), and are not at all like the small pixie-like creatures depicted in Merlin. They inhabit the Otherworld (which is similar to the Otherworld of Welsh mythology known as "Annwn" and may well be linked to "Avalon") and are immortal, but not necessarily evil. However, they do tend only to care about themselves even when dealing with men, and iron and steel are traditionally inimical to them.